Method of measuring, device manufacturing method, metrology apparatus, and lithographic system

ABSTRACT

Methods and apparatuses for measuring a plurality of structures formed on a substrate are disclosed. In one arrangement, a method includes obtaining data from a first measurement process. The first measurement process including individually measuring each of the plurality of structures to measure a first property of the structure. A second measurement process is used to measure a second property of each of the plurality of structures. The second measurement process includes illuminating each structure with radiation having a radiation property that is individually selected for that structure using the measured first property for the structure.

This application is a continuation of U.S. patent application Ser. No.15/942,423, filed Mar. 30, 2018, now allowed, which claims the benefitof priority of European patent application no. 17166691, filed Apr. 14,2017, and European patent application no. 18156860, filed Feb. 15, 2018,the content of each of the foregoing applications is incorporated hereinin its entirety by reference.

FIELD

The present description relates to methods and apparatuses for measuringa plurality of structures formed on a substrate, a device manufacturingmethod, and a lithographic system.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. In lithographic processes, itis desirable frequently to make measurements of the structures created,e.g., for process control and verification. Various tools for makingsuch measurements are known, including scanning electron microscopes,which are often used to measure critical dimension (CD), and specializedtools to measure overlay, a measure of the accuracy of alignment of twolayers in a device. Overlay may be described in terms of the degree ofmisalignment between the two layers, for example reference to a measuredoverlay of 1 nm may describe a situation where two layers are misalignedby 1 nm.

Recently, various forms of scatterometers have been developed for use inthe lithographic field. These devices direct a beam of radiation onto atarget and measure one or more properties of the scatteredradiation—e.g., intensity at a single angle of reflection, or over arange of angles of reflection, as a function of wavelength; intensity atone or more wavelengths as a function of reflected angle; orpolarization as a function of reflected angle—to obtain a “spectrum”from which a property of interest of the target can be determined.Determination of the property of interest may be performed by varioustechniques: e.g., reconstruction of the target by iterative approachesimplemented using rigorous coupled wave analysis or finite elementmethods; library searches; and principal component analysis.

Targets may be measured using dark field scatterometry in which thezeroth order of diffraction (corresponding to a specular reflection) isblocked, and only higher orders processed. Examples of dark fieldmetrology can be found in PCT patent application publication nos. WO2009/078708 and WO 2009/106279 which documents are hereby incorporatedin their entireties by reference. Further developments of the techniquehave been described in U.S. patent application publication nos. US2011-0027704, US 2011-0043791 and US 2012-0242970. The contents of allthese applications are also incorporated herein in their entireties byreference. Diffraction-based overlay using dark-field detection of thediffraction orders enables overlay measurements on smaller targets.These targets can be smaller than the illumination spot and may besurrounded by product structures on a substrate. Targets can comprisemultiple periodic structures (e.g., gratings) which can be measured inone image.

Intensity asymmetry between different diffraction orders (e.g. between−1^(st) and the +1^(st) diffraction orders) for a given overlay targetprovides a measurement of target asymmetry; that is, asymmetry in thetarget. This asymmetry in the overlay target can be used as an indicatorof overlay (e.g., undesired misalignment of two layers or misalignmentof two sets of features on a same layer).

SUMMARY

The strength of the intensity asymmetry has been observed to varybetween different substrates due to processing variations betweendifferent target structures. Variations in the thickness of thin filmstacks within target structures can affect the strength of the intensityasymmetry for example. Modelling and/or measurement errors can bereduced by changing properties of the illumination radiation such as thewavelength of the illumination radiation.

It is desirable to improve existing methods and apparatus for measuringplural target structures on a same substrate.

According to an aspect, there is provided a method of measuring aplurality of structures formed on a substrate, the method comprising:obtaining data from a first measurement process, the first measurementprocess comprising individually measuring each of the plurality ofstructures to measure a first property of the structure; and using asecond measurement process to measure a second property of each of theplurality of structures, the second measurement process comprisingilluminating each structure with radiation having a radiation propertythat is individually selected for that structure using the measuredfirst property for the structure.

According to an aspect, there is provided a metrology apparatus formeasuring a plurality of structures on a substrate, the metrologyapparatus comprising: a first measurement system configured to perform afirst measurement process, the first measurement process comprisingindividually measuring each of the plurality of structures to measure afirst property of the structure; a second measurement system configuredto perform a second measurement process, the second measurement processcomprising measuring a second property of each of the plurality ofstructures; and a controller configured to control the secondmeasurement process such that a radiation property of radiation used toilluminate each structure during the second measurement process isindividually selected for that structure using the measured firstproperty for the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus;

FIG. 2 depicts a lithographic cell or cluster;

FIG. 3A is a schematic diagram of a metrology apparatus for use inmeasuring targets using a first pair of illumination apertures;

FIG. 3B is a schematic detail of diffraction spectrum of a targetperiodic structures for a given direction of illumination;

FIG. 3C is a schematic depiction of a form of multiple periodicstructure target and an outline of a measurement spot on a substrate;

FIG. 3D is a schematic depiction of an image of the target of FIG. 3Cobtained in the metrology apparatus of FIG. 3A;

FIG. 4 depicts an embodiment of a target structure with an example ofbottom grating asymmetry;

FIG. 5 depicts an example modeling of the target structure of FIG. 4 bysplitting the asymmetric lower periodic structure into two phase-shiftedsymmetric periodic structures;

FIG. 6 depicts a metrology apparatus comprising a first measurementsystem, second measurement system, and control system;

FIG. 7 depicts simulated curves of overlay sensitivity K againstwavelength λ of measurement radiation (swing curves) for targetstructures having different thin film stack differences;

FIG. 8 is a graph depicting correlation between wavelengths λ_(p)corresponding to peak positions in swing curves (e.g., optimumwavelengths) and signal strength I₀ from a sensor (e.g., a focussensor);

FIG. 9 is a graph comparing the results of overlay measurements that usea fixed wavelength to measure overlay in all target structures (starsymbols) and the results of overlay measurements in which the wavelengthis adapted for each target structure individually (circle symbols);

FIG. 10 is a graph depicting selection of wavelengths for a dualwavelength metrology method;

FIG. 11 is a graph depicting correlation between a sensitivity measuredat a first wavelength of a dual wavelength metrology method and anoptimal value for a second wavelength; and

FIG. 12 is a graph depicting selection of a second wavelength for a dualwavelength metrology method based on sensitivity measured at anintermediate wavelength.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatefeatures of this invention. The disclosed embodiment(s) merely exemplifythe invention. The scope of the invention is not limited to thedisclosed embodiment(s). The invention is defined by the claims appendedhereto.

The embodiment(s) described, and references in the specification to “oneembodiment,” “an embodiment,” “an example embodiment,” etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a supportstructure (e.g., a mask table) MT constructed to support a patterningdevice (e.g., a mask) MA and connected to a first positioner PMconfigured to accurately position the patterning device in accordancewith certain parameters, a substrate table (e.g., a wafer table) WTconstructed to hold a substrate (e.g., a resist coated wafer) W andconnected to a second positioner PW configured to accurately positionthe substrate in accordance with certain parameters, and a projectionsystem (e.g., a refractive projection lens system) PS configured toproject a pattern imparted to the radiation beam B by patterning deviceMA onto a target portion C (e.g., comprising one or more dies) of thesubstrate W. The projection system may be supported on a reference frameRF (which can hold one or more metrology apparatuses), wherein thereference frame may be supported on a base frame BF.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, includingrefractive, reflective, catadioptric, magnetic, electromagnetic andelectrostatic optical systems, or any combination thereof, asappropriate for the exposure radiation being used, or for other factorssuch as the use of an immersion liquid or the use of a vacuum. Any useof the term “projection lens” herein may be considered as synonymouswith the more general term “projection system.”

In this embodiment, for example, the apparatus is of a transmissive type(e.g., employing a transmissive mask). Alternatively, the apparatus maybe of a reflective type (e.g., employing a programmable mirror array ofa type as referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables and, for example, two or more patterning devicetables. In such “multiple stage” machines the additional tables may beused in parallel, or preparatory steps may be carried out on one or moretables while one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for examplebetween the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (which are commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam to have a desired uniformity and intensitydistribution in its cross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g., aninterferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g., so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and another position sensor (whichis not explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g., after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-) magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2 the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes one or more apparatuses to perform pre- andpost-exposure processes on a substrate. Conventionally these include oneor more spin coaters SC to deposit resist layers, one or more developersDE to develop exposed resist, one or more chill plates CH and/or one ormore bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatuses and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU that is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatuses can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure one or more properties such asoverlay (e.g., between subsequent layers), line thicknesses, criticaldimensions (CD), etc. If errors are detected, adjustments, for example,can be made to exposures of subsequent substrates, especially if theinspection can be done soon and fast enough that other substrates of thesame batch are still to be exposed. Also, already exposed substrates maybe stripped and reworked to improve yield, or possibly be discarded,thereby avoiding performing exposures on substrates that are known to befaulty. In a case where only some target portions of a substrate arefaulty, further exposures can be performed only on those target portionsthat are deemed to be non-faulty.

A metrology apparatus is used to determine the one or more properties ofthe substrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The metrology apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the metrologyapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast, as in there is only a very small difference in refractiveindex between the parts of the resist which have been exposed toradiation and those which have not—and not all metrology apparatus havesufficient sensitivity to make useful measurements of the latent image.Therefore measurements may be taken after the post-exposure bake step(PEB) that is customarily the first step carried out on exposedsubstrates and increases the contrast between exposed and unexposedparts of the resist. At this stage, the image in the resist may bereferred to as semi-latent. It is also possible to make measurements ofthe developed resist image, at which point either the exposed orunexposed parts of the resist have been removed, or after a patterntransfer step such as etching. The latter possibility limits thepossibilities for rework of faulty substrates but may still provideuseful information.

An example metrology apparatus is shown in FIG. 3A. A target T anddiffracted rays of measurement radiation used to illuminate the targetare illustrated in more detail in FIG. 3B. The metrology apparatusillustrated is of a type known as a dark field metrology apparatus. Themetrology apparatus may be a stand-alone device or incorporated ineither the lithographic apparatus LA, e.g., at the measurement station,or the lithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. In thisapparatus, radiation emitted by source 11 (e.g., a xenon lamp) isdirected onto substrate W via a beam splitter 15 by an optical systemcomprising lenses 12, 14 and objective lens 16. These lenses arearranged in a double sequence of a 4F arrangement. A different lensarrangement can be used, provided that it still provides a substrateimage onto a detector, and simultaneously allows for access of anintermediate pupil-plane for spatial-frequency filtering. Therefore, theangular range at which the radiation is incident on the substrate can beselected by defining a spatial intensity distribution in a plane thatpresents the spatial spectrum of the substrate plane, here referred toas a (conjugate) pupil plane. In particular, this can be done byinserting an aperture plate 13 of suitable form between lenses 12 and14, in a plane which is a back-projected image of the objective lenspupil plane. In the example illustrated, aperture plate 13 has differentforms, labeled 13N and 13S, allowing different illumination modes to beselected. The illumination system in the present examples forms anoff-axis illumination mode. In the first illumination mode, apertureplate 13N provides off-axis from a direction designated, for the sake ofdescription only, as ‘north’. In a second illumination mode, apertureplate 13S is used to provide similar illumination, but from an oppositedirection, labeled ‘south’. Other modes of illumination are possible byusing different apertures. The rest of the pupil plane is desirably darkas any unnecessary radiation outside the desired illumination mode willinterfere with the desired measurement signals.

As shown in FIG. 3B, target T is placed with substrate W normal to theoptical axis O of objective lens 16. The substrate W may be supported bya support (not shown). A ray of measurement radiation I impinging ontarget T from an angle off the axis O gives rise to a zeroth order ray(solid line 0) and two first order rays (dot-chain line +1 and doubledot-chain line −1). It should be remembered that with an overfilledsmall target, these rays are just one of many parallel rays covering thearea of the substrate including metrology target T and other features.Since the aperture in plate 13 has a finite width (necessary to admit auseful quantity of radiation, the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown. Note that the periodic structure pitches ofthe targets and the illumination angles can be designed or adjusted sothat the first order rays entering the objective lens are closelyaligned with the central optical axis. The rays illustrated in FIG. 3Aand FIG. 3B are shown somewhat off axis, purely to enable them to bemore easily distinguished in the diagram.

At least the 0 and +1 orders diffracted by the target T on substrate Ware collected by objective lens 16 and directed back through beamsplitter 15. Returning to FIG. 3A, both the first and secondillumination modes are illustrated, by designating diametricallyopposite apertures labeled as north (N) and south (S). When the incidentray I of measurement radiation is from the north side of the opticalaxis, that is when the first illumination mode is applied using apertureplate 13N, the +1 diffracted rays, which are labeled +1(N), enter theobjective lens 16. In contrast, when the second illumination mode isapplied using aperture plate 13S the −1 diffracted rays (labeled −1(S))are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for manymeasurement purposes such as reconstruction.

In the second measurement branch, optical system 20, 22 forms an imageof the target T on sensor 23 (e.g. a CCD or CMOS sensor). In the secondmeasurement branch, an aperture stop 21 is provided in a plane that isconjugate to the pupil-plane. Aperture stop 21 functions to block thezeroth order diffracted beam so that the image of the target formed onsensor 23 is formed only from the −1 or +1 first order beam. The imagescaptured by sensors 19 and 23 are output to processor PU which processesthe image, the function of which will depend on the particular type ofmeasurements being performed. Note that the term ‘image’ is used here ina broad sense. An image of the periodic structure features as such willnot be formed, if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiment of the invention,on-axis illumination of the targets is used and an aperture stop with anoff-axis aperture is used to pass substantially only one first order ofdiffracted radiation to the sensor. In yet other embodiments, 2^(nd),3^(rd) and higher order beams (not shown in FIG. 3) can be used inmeasurements, instead of or in addition to the first order beams.

In order to make the measurement radiation adaptable to these differenttypes of measurement, the aperture plate 13 may comprise a number ofaperture patterns formed around a disc, which rotates to bring a desiredpattern into place. Note that aperture plate 13N or 13S can only be usedto measure periodic structures oriented in one direction (X or Ydepending on the set-up). For measurement of an orthogonal periodicstructure, rotation of the target through 90° and 270° might beimplemented. The use of these, and numerous other variations andapplications of the apparatus are described in prior publishedapplications, mentioned above.

FIG. 3C depicts a (composite) target formed on a substrate according toknown practice. The target in this example comprises four periodicstructures 25 a to 25 d positioned closely together so that they willall be within a measurement scene or measurement spot 24 formed by themetrology radiation illumination beam of the metrology apparatus. Thefour periodic structures thus are all simultaneously illuminated andsimultaneously imaged on sensors 19 and 23. In an example dedicated tomeasurement of overlay, periodic structures 25 a to 25 d are themselvescomposite periodic structures formed by overlying periodic structuresthat are patterned in different layers of the device formed on substrateW. Periodic structures 25 a to 25 d may have differently biased overlayoffsets (deliberate mismatch between layers) in order to facilitatemeasurement of overlay between the layers in which the different partsof the composite periodic structures are formed. Such techniques arewell known to the skilled person and will not be described further.Periodic structures 25 a to 25 d may also differ in their orientation,as shown, so as to diffract incoming radiation in X and Y directions. Inone example, periodic structures 25 a and 25 c are X-direction periodicstructures with biases of the +d, −d, respectively. Periodic structures25 b and 25 d are Y-direction periodic structures with offsets +d and −drespectively. Separate images of these periodic structures can beidentified in the image captured by sensor 23. This is only one exampleof a target. A target may comprise more or fewer than four periodicstructures, or only a single periodic structure.

FIG. 3D shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 3C in the apparatus of FIG.3A. While the pupil plane image sensor 19 cannot resolve the differentindividual periodic structures 25 a to 25 d, the image sensor 23 can doso. The dark rectangle represents the field of the image on the sensor,within which the illuminated spot 24 on the substrate is imaged into acorresponding circular area 26. Within this, rectangular areas 27 a to27 d represent the images of the small target periodic structures 25 ato 25 d. If the targets are located in product areas, product featuresmay also be visible in the periphery of this image field. Imageprocessor and control system PU processes these images using patternrecognition to identify the separate images 27 a to 27 d of periodicstructures 25 a to 25 d. In this way, the images do not have to bealigned very precisely at a specific location within the sensor frame,which greatly improves throughput of the measuring apparatus as a whole.

Once the separate images of the periodic structures have beenidentified, the intensities of those individual images can be measured,e.g., by averaging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter.

A typical target structure for overlay measurement comprises twoperiodic structure (e.g., gratings) formed in different layers at a samelocation on the substrate W. The two periodic structures are separatedby one or more thin films (a thin film stack) that are deposited as partof the device structure being manufactured. For example, a typical DRAMmanufacturing process uses a series of oxide and nitride thin filmsbetween the lower and upper periodic structures, with the upper periodicstructure being printed on an anti-reflection and hardmask thin film.

The thickness of the thin film stack in each target structure variesaccording to the position of the target structure due to processingvariations. The thickness of the thin film stack has a direct impact onthe reflectance properties of the thin film stack. Processing such aschemical mechanical planarization and/or etching may additionally causeasymmetry in the lower periodic structure. This is generally referred toas “bottom grating asymmetry”. The bottom grating asymmetry causesfurther variation in reflectance properties. The bottom gratingasymmetry is particularly problematic because it contributesasymmetrically to detected intensities and therefore contributes toerrors in measurements or modelling that rely on asymmetry, such as forobtaining overlay.

The strength of the intensity asymmetry from the property of interest(e.g. overlay) depends on one or more properties of the measurementradiation used. The strength of the intensity asymmetry may vary, forexample, as a function of the central wavelength, bandwidth and/orpolarization of the measurement radiation. The strength of the intensityasymmetry may be referred to as sensitivity. A curve of predictedsensitivity against a property of interest may be generated. Such curvesare known as swing curves. For high accuracy it is desirable to selectone or more properties of the measurement radiation which correspond topeaks or valleys in the swing curves. Measuring at, e.g., a peak of aswing curve may improve the accuracy with which the property of interestis obtained because modelling errors may be reduced (formulae used forobtaining overlay, for example, may be more accurate at the peaks of theswing curves). Furthermore, measuring at the peaks of the swing curvemay provide a stronger signal, which makes measurements more robustagainst noise.

An optimum property of the measurement radiation may not be invariantover the substrate W. This may arise for example due to processvariations across the substrate W that lead to variations in the targetstructures, for example differences in the thickness of the thin filmstack between the upper and lower periodic structures and/or differencesin bottom grating asymmetry. As described in detail below, embodimentsare disclosed which allow errors in metrology measurements to be furtherreduced relative to current approaches by varying one or more propertiesof the measurement radiation adaptively as a function of the location ofthe target structure to be measured on the substrate W. The measurementradiation is individually tuned on a target structure by targetstructure basis rather than on a substrate by substrate basis. In anembodiment for overlay target structures, reduced overlay errors (errorsin overlay measurements) are observed.

As a detailed illustration of the concept, the discussion belowdemonstrates how the effect of bottom grating asymmetry can be minimizedby selecting a wavelength of the measurement radiation to be at a peakof a sensitivity curve on a target by target basis. Due to processvariations, the position of the peak of the sensitivity curve varies asa function of the location of the target structure on the substrate Wdue to variations in the thin film stack between the upper and lowerperiodic structures of the target structures.

FIGS. 4 and 5 depict how a target structure 30 can be modeled. Asdepicted in FIG. 4, the target structure 30 comprises an upper periodicstructure (e.g., grating) 31 and a lower periodic structure (e.g.,grating) 32. The upper periodic structure 31 is separated from the lowerperiodic structure 32 by a thin film stack 33. The lower periodicstructure 32 is deformed due to bottom grating asymmetry. FIG. 5 depictshow the target structure 30 can be modeled by notionally splitting theasymmetric lower periodic structure 32 into two separate periodicstructures 32A and 32B, displaced laterally relative to each other toapproximate the asymmetry.

The diffracted signals from the upper periodic structure 31 and each ofthe two lower periodic structures 32A and 32B can be rigorously solvedusing Maxwell's equations. An intuitive understanding can be obtained,however, using a scalar approach with some approximations, as describedbelow.

The total signal can be expressed as the sum of the diffracted wavesfrom the upper periodic structure 31 and from each of the two lowerperiodic structures 32A, 32B. The +1^(st) and −1^(st) orders diffractedby the upper periodic structure 31 can be written as Ae^(iα) andAe^(−iα) where A is the amplitude, α is the position dependent phaseterm, given by α=2πOV/P with OV being the overlay term, and P being thepitch of the periodic structure.

The diffraction from the lower periodic structures 32A, 32B can beexpressed similarly as Be^(iβ)e^(iγ)+Be^(iβ)e^(iδ)Ce^(iη) as the +1^(st)order and Be^(iβ)e^(−iγ)+Be^(iβ)e^(iδ)Ce^(−iΘ) as the −1^(st) order,where is the thickness induced phase acquired during propagation throughthe thin film of thickness d, given by

${\beta = \frac{4\pi\;{nd}}{\lambda}},$with n being the refractive index of the thin film separating theperiodic structures, λ being the wavelength for measurement, δ being theadditional phase acquired due to the extra propagation till thelowermost lower periodic structure 32B, and η being the phase due to theshift of the lowermost lower periodic structure 32B (i.e. the bottomgrating asymmetry) and C being the amplitude of the diffracted wave fromthe lowermost lower periodic structure 32B.

The total electric field of the +1^(st) order due to the combination ofthese three diffracted waves is given byE ₊₁ =Ae ^(iα) +Be ^(iβ) e ^(iγ) +Be ^(iβ) e ^(iδ) Ce ^(iη)

Since the intensity is detected, the total intensity of the +1^(st)order detected isI ⁻¹ =|Ae ^(iα) +Be ^(iβ) e ^(iγ) +Be ^(iβ) e ^(iδ) Ce ^(iη)|²and the intensity of the −1^(st) order isI ⁻¹ =|Ae ^(−iα) +Be ^(iβ) e ^(−iγ) +Be ^(iβ) e ^(iδ) Ce ^(−iη)|²

These expressions can be written as follows:

I₊₁ = A² + B² + B^(′)² + 2ABcos (α − (β + γ)) + 2BB^(′)cos (γ − (δ + η)) + 2ABcos (α − (β + δ + η))I⁻¹ = A² + B² + B^(′)² + 2ABcos (−α − (β + γ)) + 2BB^(′)cos (−γ − (δ − η)) + 2ABcos (−α − (β + δ − η))with |B′|=|B∥C| and the resulting overlay signal being the difference inintensities, given byΔI=4|A∥B|sin β sin α−4|B∥B′|sin η sin δ+4|A∥B′|sin(β+δ)sin(α−η)

α is small because typical overlay numbers are much smaller than thepitch of the periodic structure used, which means that αI can be writtenasΔI≈α(4|A∥B|sin β+4|A∥B′|sin(β+δ))−4|A∥B′|sin(β+δ)η+4|B∥B′|sin η sin δ

When two overlay targets are biased with respect to each other by a biasd, ΔI for each of the two biases are given by

${\Delta\; I_{+ d}} = {{\left( {\alpha + \frac{2\pi\; d}{P}} \right)\left( {{4{A}{B}\sin\;\beta} + {4{A}{B^{\prime}}{\sin\left( {\beta + \delta} \right)}}} \right)} - {4{A}{B^{\prime}}{\sin\left( {\beta + \delta} \right)}\eta} + {4{B}{B^{\prime}}\sin\;{\eta sin}\;\delta}}$${\Delta\; I_{- d}} = {{\left( {\alpha - \frac{2\pi\; d}{P}} \right)\left( {{4{A}{B}\sin\;\beta} + {4{A}{B^{\prime}}{\sin\left( {\beta + \delta} \right)}}} \right)} - {4{A}{B^{\prime}}{\sin\left( {\beta + \delta} \right)}\eta} + {4{B}{B^{\prime}}\sin\;{\eta sin}\;\delta}}$with the overlay being calculated by the formula

${OV}_{r} = {2d\frac{{\Delta\; I_{+ d}} + {\Delta\; I_{- d}}}{{\Delta\; I_{+ d}} - {\Delta\; I_{- d}}}}$${OV}_{r} = {{OV} + {2d\frac{{4{B}{B^{\prime}}\sin\;{\eta sin}\;\delta} - {4{A}{B^{\prime}}{\sin\left( {\beta + \delta} \right)}\eta}}{{4{A}{B^{\;}}\sin\;\beta} + {4{A}{B^{\prime}}{\sin\left( {\beta + \delta} \right)}}}}}$${OV}_{r} \cong {{OV} + {2d\frac{4\eta{B^{\prime}}\left( {{{B}\sin\;\delta} - {4{A}{\sin\left( {\beta + \delta} \right)}}} \right)}{{4{A}{B}\sin\;\beta} + {4{A}{B^{\prime}}{\sin\left( {\beta + \delta} \right)}}}}}$

The above equation shows that the retrieved overlay is the true overlaywith an error term. The error term can be minimized when |A|≅|B| i.e.,when the periodic structures are balanced in diffraction efficiency.This can be achieved by target design optimization.

The error term can also be reduced by maximizing the denominator term.The denominator is similar to the ΔI term discussed above. This meansthat if the measurement wavelength is chosen such that the intensitydifference between the two orders is maximized, the error due to thebottom grating asymmetry will be minimized. Thus, the error over alllocations on the substrate W can be minimized by adjusting thewavelength of measurement radiation to stay at the peak of the curve ofsensitivity against wavelength (the swing curve).

It has been recognized that it is possible to predict how much thewavelength needs to be adjusted before the overlay measurement by usinginformation derived from a separate sensor. A focus sensor canconveniently be used to provide the necessary information, for example.As described below, this is possible because the focus sensor issensitive to the same thin film variations which affect the overlayerror.

FIG. 6 depicts an example metrology apparatus based on the aboveprinciples. The metrology apparatus comprises a first measurement system61 and a second measurement system 62. The metrology apparatus may beprovided as part of a lithographic system, for example as describedabove with reference to FIGS. 1 and 2. The metrology apparatus isconfigured to measure a plurality of structures on a substrate W. In anembodiment the plurality of structures are formed on the substrate W bya lithographic process. In an embodiment the plurality of structurescomprise target structures for measuring a parameter of a lithographicprocess, such as overlay. The metrology apparatus may be used in adevice manufacturing method comprising forming the plurality of thestructures using lithography and measuring the plurality of structuresusing the metrology apparatus.

The first measurement system 61 performs a first measurement process.The first measurement process comprises individually measuring each ofthe plurality of structures to measure a first property of thestructure. In an embodiment, the first measurement system 61 comprises afirst radiation source 42. The first radiation source 42 illuminateseach structure with radiation 55 via an optical system 44.

The second measurement system 62 performs a second measurement process.The second measurement process comprises measuring a second property ofeach of the plurality of structures. In an embodiment, the secondmeasurement system 62 comprises a second radiation source 11. The secondradiation source 11 also illuminates each structure with radiation. Inan embodiment, the first radiation source 42 is different from thesecond radiation source 11, for example configured to output radiationhaving one or more different properties and/or housed in a separatedevice. The radiation from the first radiation source 42 is configuredto be suitable for performing the first measurement process. Theradiation from the second radiation source 11 is configured to besuitable for performing the second measurement process.

The second measurement system 62 comprises an optical system 40configured to direct radiation 51 from the first radiation source 11onto the substrate W as incident radiation 52A. Redirected radiation 52Bfrom the substrate W is directed by the optical system 40 onto one ormore sensors 19,23. In an embodiment, the second measurement system 62comprises a metrology apparatus of the type described above withreference to FIG. 3. In embodiments of this type, the optical system 40may comprise lenses 12 and 14 and an objective lens 16, as depicted inFIG. 3A. The optical system 40 may further comprise a beam splitter 15configured to direct the radiation 51 towards the substrate W, asdepicted in FIG. 3A. The optical system 40 may further comprise eitheror both of the first measurement branch and the second measurementbranch. In the particular example of FIG. 6, both of these measurementbranches are provided. Example details of the optical elements of eachof the measurement branches are depicted in FIG. 3A. An output 53 fromthe first measurement branch is directed to the sensor 19. An output 54from the second measurement branch is directed to the sensor 23.

In an embodiment, the second property of each structure measured by thesecond measurement process comprises overlay (e.g., undesiredmisalignment between different layers of the structure).

In an embodiment, the first measurement system 61 comprises one or moreoptical elements that are also used by a focus measurement systemconfigured to measure a focus of the optical system 40 used by thesecond measurement system 62. Focus measurement systems are commonlyincorporated into metrology apparatus to allow target structures to bealigned and/or brought to focus prior to measurements being performedthat use the target structures. In an embodiment, the one or moreoptical elements are not used by the second measurement system 62. Inthe example of FIG. 6, the first measurement system 61 uses an opticalsystem 44 of the focus measurement system and a focus sensor 46 of thefocus measurement system. Radiation from the first radiation source 42(which may or may not be the same radiation source that is used when thefocus measurement system is measuring focus) is directed via the opticalsystem 44 and the optical system 40 onto the substrate W. In anembodiment, the optical system 40 comprises a further beam splitter aspart of the objective lens 16 (see FIG. 3A) to direct radiation from thefirst radiation source 42 from the optical system 44 to the substrate Wand back from the substrate W to the optical system 44. The firstmeasurement process uses an output from the focus sensor 46. In anembodiment, the first property of each structure measured by the firstmeasurement process comprises reflectivity and the signal strength fromthe focus sensor 46 is used to determine the reflectivity. In anembodiment, a dedicated first measurement system is provided whoseprimary purpose is to provide information for optimizing the radiationused in the second measurement process. Other sensing schemes can beused, including for example ellipsometric or spectroscopic measurementmodes. Using such sensing schemes, the first property of the structuremay additionally or alternatively comprise an effect of the structure onthe polarization of radiation scattered from the structure.

In an embodiment, a control system 48 controls the second measurementprocess such that a radiation property of radiation used to illuminateeach structure during the second measurement process is individuallyselected for that structure using the measured first property for thestructure. In an embodiment, a spectral distribution of intensity of theradiation is individually selected. The spectral distribution maycomprise either or both of the central wavelength and/or bandwidth ofthe radiation. Alternatively or additionally, polarization of theradiation is individually selected. In an embodiment, the individualselection of the radiation property (e.g. wavelength) for the secondmeasurement process for each structure is performed based on apreviously measured correlation between the first property (e.g.reflectivity) and a choice of the radiation property (e.g. wavelength)for the second measurement process that enables a performance of thesecond measurement process (e.g. overlay measurement) to be higher thanfor other choices of the radiation property (e.g. wavelength) for thesecond measurement process.

Details are given below about how this can be achieved in the particularcase where a signal strength from a focus sensor 46 is used to optimizea wavelength used in an overlay measurement. Reference is made to themathematical analysis discussed above with reference to FIGS. 4 and 5.

In a typical focus measurement system, the focus sensor 46 can measurethe total reflection from the substrate W with a large illumination NA.The focus sensor 46 can also detect the normally reflected radiationintensity. The expected relationship between the reflected 0th ordersignal and the measured overlay signal ΔI is described below.

The 0^(th) order signal detected by the focus sensor 46 can be expressedas the sum of all the reflected and diffracted waves at the focus sensorwavelength. For simplicity, the following discussion considers only thereflection at normal incidence.

The reflected waves at normal incidence from the upper periodicstructure 31 and lower periodic structure 32A, 32B can be expressed as

$I_{0} = {{{A_{t} + {A_{b}e^{\frac{i\; 4\;\pi\;{dn}}{\lambda_{f}}}}}}^{2} = {{A_{t}^{2} + A_{b}^{2} + {2A_{t}A_{b}\cos\frac{4\pi\;{nd}}{\lambda_{f}}}} = {{\hat{A} + {\hat{B}\;\cos\frac{\varphi}{\lambda_{f}}}} = {\hat{A} + {\hat{B}\;\cos\;{\varphi \cdot v_{f}}}}}}}$where φ=4πnd, λ_(f) is the focus sensor wavelength, and ν_(f) is theequivalent frequency.

As shown in the overlay signal analysis above, the ΔI term has apredominant wavelength dependence term

${\Delta\; I} = {{4{A}{B}\sin\;{\beta \cdot \alpha}} = {{4{A}{B}\sin\;{\frac{4\pi\;{nd}}{\lambda_{m}} \cdot \alpha}} = {\hat{C}\;\sin\;{\varphi \cdot v_{m}}}}}$where φ=4πnd, λ_(m) is the metrology (overlay) measurement wavelength,and ν_(m) is the equivalent frequency. The other constants are writtenas Ĉ.

The signal from the focus sensor 46 and the overlay signal are bothdependent on the thin film thickness, d. This means that the variationin the thin film thickness, d, can be detected in the signal strength ofthe focus sensor 46 and appropriate corrections can be made to thewavelength used in the second measurement process (for measuringoverlay).

Writing ν_(f)=v_(m)+Δv, where Δv is the frequency separation, the signalI₀ can be written as

$\begin{matrix}{I_{0} = {\hat{A} + {B\;\cos\;{\varphi \cdot v_{f}}}}} \\{= {\hat{A} + {\hat{B}\;\cos\;{\varphi\left( {\upsilon_{m} + {\Delta\;\upsilon}} \right)}}}} \\{= {\hat{A} + {\hat{B}\;\left( {{\cos\;{\varphi \cdot \upsilon_{m}}\cos\;\Delta\;\upsilon} - {\sin\;{\varphi \cdot \upsilon_{m}}\sin\;\Delta\;\upsilon}} \right.}}} \\{= {\hat{A} + {\hat{B}\left( {{\sqrt{1 - {\sin^{2}\left( {\varphi \cdot \upsilon_{m}} \right)}}\cos\;\Delta\;\upsilon} - {\sin\;{\varphi \cdot \upsilon_{m}}\sin\;{\varphi \cdot \Delta}\;\upsilon}} \right.}}}\end{matrix}$$I_{0} = {\hat{A} + {\hat{B}\sqrt{1 - \frac{\Delta\; I^{2}}{{\hat{C}}^{2}}}\cos\;{\varphi \cdot \Delta}\;\upsilon} - {\frac{\Delta\; I}{\hat{C}}\sin\;{\varphi \cdot \Delta}\;\upsilon}}$

Thus, the relationship between the signal strength from the focus sensor46 and the sensitivity of the overlay signal can be expressed as aquadratic relationship. The signal strength of the focus sensor 46 cantherefore be used to estimate how much the wavelength of the radiationused for the second measurement process needs to be adjusted.

The output from the focus sensor 46 can be used to infer variations ofthe target structure 30 (e.g. variations in the thickness of the thinfilm stack 33) that will affect the sensitivity of the secondmeasurement process. In an embodiment, a shift in the curve of overlaysensitivity against wavelength (swing curve) is determined. Thewavelength of the radiation used for the second measurement process canthen be shifted by the same amount so that the second measurementprocess can be performed at the peak of the swing curve.

An example focus sensor 46 operates using radiation at two differentwavelengths (e.g., 670 nm and 785 nm). The focus sensor 46 forms aradiation spot on the substrate W of, e.g., around 7 μm in size. Thetarget structure 30 may be configured so that the radiation spotunderfills the target structure 30. This means that the signal to thefocus sensor 46 will not be corrupted by product structures outside ofthe target structure 30. The total reflected signal will also beindependent of the overlay because the total reflected intensity isdetected (all reflected orders).

When there is a process variation, the absolute reflectivity of eachtarget structure 30 changes. It has been found that the absolutereflectivity at the two wavelengths is directly correlated with theoptimum wavelength to use for measuring overlay in each target structure30 (e.g., the peak of the swing curve).

In this example, the following steps can be used to adjust thewavelength of the second measurement process. In a first step, the focussensor 46 is used to measure the absolute total reflectivities of thetarget structure 30 at each of the two wavelengths available. The totalreflectivity can be measured for example by bringing the substrate W tooptimum focus. At optimum focus, the signal strength from the focussensor 46 is maximal. The signal strength from the focus sensor 46 isobserved to vary as a function of position over the substrate W,reflecting variations in the properties of the thin film stack 33 overthe substrate W. In a second step, the absolute reflectivity is comparedwith a previously measured correlation between the absolute reflectivityand an optimum choice of wavelength to determine a shift in the swingcurve. In a third step, an output from the second radiation source 11 ofthe second measurement system 62 is adjusted prior to measurement of thetarget structure 30 using the second measurement system 62 (e.g. toobtain overlay).

Simulations have been performed to demonstrate the effectiveness of thetechnique. The simulations were performed by changing thin film stackthicknesses randomly and calculating the effect on the swing curves ineach case. FIG. 7 depicts simulated swing curves of overlay sensitivityK against radiation wavelength λ for different thin film stackthicknesses (corresponding to target structures 30 located at differentpositions on the substrate W). The peak positions of the swing curvesare marked with a circular mark and are spread over a range ofwavelengths. The ideal wavelength to use for the overlay measurementtherefore varies between different target structures 30. FIG. 8 showshow the wavelength λ_(p) corresponding to each peak position correlatesquadratically (almost linearly) with a signal strength I₀ from the focussensor 46. The signal strength I₀ from the focus sensor 46 can thereforebe used to determine the shift in the swing curve and allow optimalselection of the wavelength for overlay measurement for the targetstructure that has been measured by the focus sensor 46.

FIG. 9 compares the results of overlay measurements that use a fixedwavelength (520 nm in this example) to measure overlay in all targetstructures 30 (star symbols) and the results of overlay measurements inwhich the wavelength is adapted for each target structure 30individually (circle symbols) using the focus sensor 46. FIG. 9 shows atotal improvement (˜7 nm) in final overlay if the illuminatingwavelength is corrected before the measurement. The fixed wavelengthapproach has large outliers which are not present in the adaptivewavelength approach. It shows that the overlay error (i.e. the accuracyof the overlay measurement) can be improved by adjusting the centralwavelength on a target by target basis. The improvement will be muchlarger for thick stacks where typically larger variations are present inthe thin film thicknesses over the substrate.

The dependence between the optimum wavelength and the focus sensorsignal can be modelled with a quadratic fit. The parameters of this fitcan be calculated based on measurements of swing curves for differentfocus sensor signals during a calibration and recipe creation step andthe results stored in a database.

In the detailed example discussed above, only two wavelengths wereavailable. In an embodiment, the first measurement system 61 comprises afirst radiation source 42 that illuminates each structure with broadbandradiation and the control system 48 performs the individual selection ofthe radiation property for the second measurement process for eachstructure based on a spectroscopic analysis of the data from the firstmeasurement process. This approach can provide more information aboutthe optimum wavelength and further improve performance. Embodiments ofthis type could be implemented by providing a dedicated firstmeasurement system or by modifying a focus measurement system such asthat discussed above. A beam splitter could be used for example todirect the broadband radiation into the optical system 44 of the focusmeasurement system. A multimode fiber could be used to direct radiationreflected back through the optical system 44 to an appropriatespectrometer. The fiber could be provided at either or both of twopinholes for receiving the two wavelengths of the particular focusmeasurement system discussed above.

In a further embodiment, the focus measurement system could be convertedto operate as an ellipsometer. This would allow measured polarizationchanges in the reflected radiation to be used to contribute to theestimation of an optimal wavelength.

In further embodiments, the first measurement process comprises one ormore sub-processes. Thus, for example, instead of the first measurementprocess measuring reflectivity only using a focus or other sensor, thefirst measurement process may measure reflectivity using, e.g., thefocus sensor (in one sub-process) and another property of the structureusing the focus sensor or a different sensor (in another sub-process).The sub-processes may comprise at least one sub-process configured tomeasure a first property of the structure and at least one sub-processconfigured to measure a second property of the structure. A combinationof the outputs from the plural sub-processes may be used to allow theindividual selection per structure of the radiation property of thesecond measurement process to be performed even more efficiently.

In some embodiments of this type, the second property of the structure(e.g. overlay) is obtained via separate measurements of the structure attwo different wavelengths. This approach may be referred to as dualwavelength metrology. In the case where the second property of thestructure comprises overlay, the approach may be referred to as dualwavelength overlay metrology. In such dual wavelength metrology methods,one of the sub-processes of the first measurement process may compriseone of the two measurements at different wavelengths. The individualselection per structure of the radiation property for the secondmeasurement process may then comprise selection of the other of the twowavelengths required for the dual wavelength metrology.

In some embodiments, the method comprises calculating a sensitivity ofeach of one or more of the sub-processes to the second property of thestructure (e.g. overlay). In such embodiments, the individual selectionper structure of the radiation property for the second measurementprocess is performed using one or more of the calculated sensitivities.This approach is based on the recognition that there is frequently asignificant correlation between the sensitivity calculated for each ofone or more of the sub-processes and an optimal radiation property (e.g.optimal wavelength) for the second measurement process. In anembodiment, the individual selection of the radiation property for thesecond measurement process is performed for each structure based on apreviously measured correlation between each of one or more of thecalculated sensitivities and a choice of the radiation property for thesecond measurement process that enables a performance of the secondmeasurement process (e.g. an accuracy with which overlay can be obtainedby the second measurement process) to be higher (e.g. more accurate)than for other choices of the radiation property for the secondmeasurement process.

Information from different measurements may be combined mathematically,for example by calculating a maximum likelihood value of an optimalradiation property (e.g. an optimal second wavelength in a dualwavelength metrology method) using the different measurement results(e.g. sensitivities calculated from different sub-processes and/ormeasurements of reflectivity obtained from a sub-process using the focussensor).

The individual selection per structure of the radiation property for thesecond measurement process may comprise selecting a central wavelengthof a spectral distribution of intensity (referred to herein simply as“wavelength” for simplicity). Alternatively or additionally, theindividual selection of the radiation property for the secondmeasurement process may comprise selecting a polarization property (e.g.a direction of linear or circularly polarized radiation). Alternativelyor additionally, the method may comprise individually selecting eitheror both of a wavelength and/or polarization property (e.g. a directionof linear or circularly polarized radiation) of redirected radiationdetected during the second measurement process using one or more of thecalculated sensitivities. Thus, either or both of wavelength and/orpolarization of incident radiation and/or redirected radiation used inthe second measurement process may be individually optimized for eachstructure on the substrate using information derived from thesub-processes of the first measurement process.

In an embodiment, the sub-processes of the first measurement processcomprise one or more selected from: illuminating the structure withradiation having a first polarization property and detecting reflectedradiation having a second polarization property; illuminating thestructure with radiation having the second polarization property anddetecting reflected radiation having the first polarization property;illuminating the structure with radiation having the first polarizationproperty and detecting reflected radiation having the first polarizationproperty; or illuminating the structure with radiation having the secondpolarization property and detecting reflected radiation having thesecond polarization property. The first polarization property isdifferent to the second polarization property. In an embodiment, thefirst polarization property is orthogonal to the second polarizationproperty (e.g. orthogonal linear polarizations or orthogonal circularpolarizations). Thus, the sub-processes may comprise different detectionmodes corresponding to different combinations of polarizations forincident and detected polarizations, including co-polarized detectionmodes and cross-polarized detection modes. The sub-processes maycomprise detection of zeroth or higher order reflectivities for anycombination of the incident and detected polarizations. Metrics may beformed from combinations of sensitivities obtained for differentdetection modes. For example, where sensitivities K_(TETM) and K_(TMTE)are obtained for two opposite cross-polarized detection modes (thesubscripts “TE” and “TM” referring to polarization modes that areorthogonal to each other), which may be opposite in sign to each other,the following metric, r, may be used:

$r = \frac{K_{TETM} - K_{TMTE}}{K_{TETM} + K_{TMTE}}$

FIG. 10 depicts an example application of the method to dual wavelengthmetrology. In embodiments of this type, at least one of thesub-processes of the first measurement process is performed using thesame measurement system (e.g. the second measurement system 62) as thesecond measurement process. The wavelength dependence of the sensitivity(e.g. the swing curve) of that sub-process for a given structure maythus be substantially the same as the wavelength dependence of thesensitivity of the second measurement process for the same structure.The dual wavelength metrology is achieved by configuring the sub-processusing the same measurement system as the second measurement process tooperate at a wavelength λ₁ and arranging for the second measurementprocess to operate at wavelength λ₂. The method then uses at least ameasurement of the sensitivity to the second property of the structuremeasured at wavelength λ₁ to select an optimal value for λ₂.

In some embodiments, it is desirable for one of λ₁ or λ₂ to be locatedat or near to a maximum of the curve of sensitivity against wavelengthand for the other of λ₁ or λ₂ to be located at or near to a minimum ofthe curve of sensitivity against wavelength. In the case where thesecond property of the structure comprises overlay OV, for example, adifference in intensity between different diffraction orders can be usedto obtain OV using the following relationship: ΔI=K₀+K₁·OV, wherein K₀is a process induced offset (independent of the structure) and K₁represents a sensitivity of the measurement process to OV. In thisparticular example, it can be shown that the error ∈ can be written asfollows:

$\epsilon = {\frac{\Delta\; K_{0}}{\Delta\; K_{1}}d}$where d is the overlay bias applied to periodic structures used in themeasurement. Choosing λ₁ and λ₂ to be on opposite sides of the curve ofsensitivity against wavelength (e.g., with one a minimum and one amaximum) ensures that ΔK₁ is large and the error is small.

FIG. 10 shows three example curves of sensitivity against wavelength.The three curves may correspond to curves measured for differentstructures on the substrate for example. The curves each have a similarform and are displaced relative to each other along the wavelengthdirection. Optimal values for λ₁ and λ₂ are different for each curve.However, it has been found (and can be seen qualitatively in FIG. 10)that there is a correlation between the sensitivity K measured at λ₁(e.g. at or near a peak of the curve of sensitivity against wavelength)and the corresponding optimal value for λ₂. The peak heights are not thesame for each of the curves. The peak height at λ₁ thus providesinformation about which curve most closely applies to the structurebeing measured and therefore provides information about where anextremum (e.g., minimum) corresponding to an optimal value for λ₂ may belocated.

In an embodiment, an optimal value for λ₁ is determined in a firstoptimization process. The first optimization process may, for example,use measurements of the structure in question using the focus sensor,for example to obtain reflectivity of the structure, according to any ofthe embodiments described above. The optimal value of λ₂ may then beobtained in a subsequent process using at least a sensitivity calculatedfrom measurements performed at the optimized value of λ₁.

FIG. 11 is a graph showing example sensitivities K_(TETE) calculatedfrom measurements performed at λ₁ plotted against corresponding optimalvalues Opt-λ₂ for λ₂ (each corresponding to a trough in the curve ofsensitivity against wavelength for the structure being measured). Inthis particular example, the measurements were performed withco-polarized, linear polarized radiation (TE polarization) for bothincident and reflected radiation.

As can be seen in FIG. 10, the change in sensitivity K at or near thepeak corresponding to λ₁ changes relatively slowly moving from one curveto the next. To increase the sensitivity with which the optimal value ofλ₂ can be derived based on the sensitivity K for λ₁, the sensitivity Kat an intermediate position between the maximum and minimum in the curveof sensitivity against wavelength can be used instead. An exampleapproach is illustrated in FIG. 12. The sensitivity K at the midpointλ_(MID) changes much more quickly moving from one curve to the nextcurve and therefore allows the optimal λ₂ to be obtained moreaccurately. An embodiment based on this effect is described below.

In an embodiment, the plurality of sub-processes comprises a firstsub-process and a second sub-process. A wavelength dependence of thesensitivity of the first sub-process to the second property (e.g.overlay) of the structure is substantially the same as the wavelengthdependence of the second sub-process to the second property of thestructure and comprises a local maximum and a local minimum (as in FIGS.10 and 12).

The first sub-process comprises illuminating the structure withradiation having a central wavelength aligned with one of the localmaximum and the local minimum, within a range of 10%, optionally 5%,optionally 1%, of the wavelength separation between the local maximumand the local minimum. Thus, the first sub-process may compriseilluminating the structure with a wavelength λ₁ at or near the peak inFIG. 12.

The second sub-process comprises illuminating the structure withradiation having a central wavelength aligned with a midpoint betweenthe local maximum and the local minimum, within a range of 40%,optionally 20%, optionally 10%, optionally 5%, optionally 1%, of thewavelength separation between the local maximum and the local minimum.Thus, the second sub-process may comprise illuminating the structurewith a wavelength at or near the estimated midpoint λ_(MID) between λ₁and λ₂ in FIG. 12. A sensitivity to the second property (e.g. overlaysensitivity) obtained at λ_(MID) is then used to derive an optimal valuefor the second wavelength λ₂, optionally in combination with informationprovided by other sub-processes of the first measurement process (e.g.reflectivity from focus sensor measurements, a sensitivity obtained whenmeasuring at λ₁ and one or more other sensitivities obtained fromsub-processes based on other detection modes).

The concepts disclosed herein may find utility beyond post-lithographymeasurement of structures for monitoring purposes. For example, such adetector architecture may be used in future alignment sensor conceptsthat are based on pupil plane detection, used in lithographicapparatuses for aligning the substrate during the patterning process.

While the targets described above are metrology targets specificallydesigned and formed for the purposes of measurement, in otherembodiments, properties may be measured on targets which are functionalparts of devices formed on the substrate. Many devices have regular,grating-like structures. The terms ‘target grating’, ‘target periodicstructure’, ‘target’ and similar wording as used herein do not requirethat the structure has been provided specifically for the measurementbeing performed.

The metrology apparatus can be used in a lithographic system, such asthe lithographic cell LC discussed above with reference to FIG. 2. Thelithographic system comprises a lithographic apparatus LA that performsa lithographic process. The lithographic apparatus may be configured touse the result of a measurement by the metrology apparatus of astructure formed by the lithographic process when performing asubsequently lithographic process, for example to improve the subsequentlithographic process.

In an embodiment, the results from methods herein may be used in thedesign, control or modification of any process or of any physical object(e.g., target, patterning device, etc.) or apparatus (e.g., metrologyapparatus) used or manufactured in the process.

An embodiment may include a computer program containing one or moresequences of machine-readable instructions describing methods ofmeasuring targets on a structures and/or analyzing measurements toobtain information about a lithographic process. There may also beprovided a data storage medium (e.g., semiconductor memory, magnetic oroptical disk) having such a computer program stored therein. Where anexisting lithography or metrology apparatus is already in productionand/or in use, an embodiment of the invention can be implemented by theprovision of updated computer program products for causing a processorto perform a method, or portion thereof, described herein.

Further embodiments according to the invention are described in belownumbered clauses:

-   1. A method of measuring a plurality of structures formed on a    substrate, the method comprising:

obtaining data from a first measurement process, the first measurementprocess comprising individually measuring each of the plurality ofstructures to measure a first property of the structure; and

using a second measurement process to measure a second property of eachof the plurality of structures, the second measurement processcomprising illuminating each structure with radiation having a radiationproperty that is individually selected for that structure using themeasured first property for the structure.

-   2. The method of clause 1, wherein the individual selection of the    radiation property for the second measurement process is performed    for each structure based on a previously measured correlation    between the first property and a choice of the radiation property    for the second measurement process that enables a performance of the    second measurement process to be higher than for other choices of    the radiation property for the second measurement process.-   3. The method of clause 1 or clause 2, wherein the first property of    the structure comprises reflectivity.-   4. The method of any preceding clause, wherein the first property of    the structure comprises an effect of the structure on the    polarization of radiation scattered from the structure.-   5. The method of any preceding clause, wherein the second property    of each structure comprises overlay between different layers of the    structure.-   6. The method of any preceding clause, wherein the radiation    property for the second measurement process comprises a spectral    distribution of intensity.-   7. The method of clause 6, wherein the spectral distribution of    intensity comprises one or both of central wavelength and bandwidth.-   8. The method of any preceding clause, wherein the radiation    property for the second measurement process comprises a polarization    of the radiation.-   9. The method of any preceding clause, wherein:

the first measurement process uses a first radiation source toilluminate each structure with radiation; and

the second measurement process uses a second radiation source toilluminate each structure with radiation, wherein the first radiationsource is different from the second radiation source.

-   10. The method of any preceding clause, wherein the first    measurement process uses the output from a focus sensor configured    to measure a focus of an optical system used for the second    measurement process.-   11. The method of clause 10, wherein the first property of the    structure comprises reflectivity and the signal strength from the    focus sensor is used to determine the reflectivity.-   12. The method of any preceding clause, wherein the first    measurement process uses one or more optical elements that are also    used when measuring a focus of an optical system used for the second    measurement process, wherein the one or more optical elements are    not used when performing the second measurement process.-   13. The method of any preceding clause, wherein the first    measurement process uses a first radiation source to illuminate each    structure with broadband radiation and the individual selection of    the radiation property for the second measurement process is    performed for each structure based on a spectroscopic analysis of    the data from the first measurement process.-   14. The method of any preceding clause, comprising performing the    first measurement process.-   15. The method of clause 1, wherein:

the first measurement process comprises one or more sub-processes;

the method comprises calculating a sensitivity of each of one or more ofthe sub-processes to the second property of the structure; and

the individual selection of the radiation property for the secondmeasurement process is performed using one or more of the calculatedsensitivities.

-   16. The method of clause 15, wherein the individual selection of the    radiation property for the second measurement process is performed    for each structure based on a previously measured correlation    between each of one or more of the calculated sensitivities and a    choice of the radiation property for the second measurement process    that enables a performance of the second measurement process to be    higher than for other choices of the radiation property for the    second measurement process.-   17. The method of clause 15 or clause 16, wherein the individual    selection of the radiation property for the second measurement    process comprises selecting a central wavelength of a spectral    distribution of intensity.-   18. The method of any of clauses 15-17, wherein the individual    selection of the radiation property for the second measurement    process comprises selecting a polarization property.-   19. The method of any of clauses 15-18, further comprising    individually selecting a polarization property of reflected    radiation detected during the second measurement process using one    or more of the calculated sensitivities.-   20. The method of any of clauses 15-19, wherein the sub-processes    comprise one or more selected from:

illuminating the structure with radiation having a first polarizationproperty and detecting reflected radiation having a second polarizationproperty;

illuminating the structure with radiation having the second polarizationproperty and detecting reflected radiation having the first polarizationproperty;

illuminating the structure with radiation having the first polarizationproperty and detecting reflected radiation having the first polarizationproperty; and illuminating the structure with radiation having thesecond polarization property and detecting reflected radiation havingthe second polarization property, wherein:

the first polarization property is different to the second polarizationproperty.

-   21. The method of clause 20, wherein the first polarization property    is orthogonal to the second polarization property.-   22. The method of any of clauses 15-21, wherein:

the sub-processes comprise a first sub-process and a second sub-process;

a wavelength dependence of the sensitivity of the first sub-process tothe second property of the structure is substantially the same as thewavelength dependence of the second sub-process to the second propertyof the structure and comprises a local maximum and a local minimum;

the first sub-process comprises illuminating the structure withradiation having a central wavelength aligned with one of the localmaximum and the local minimum, within a range of 10% of the wavelengthseparation between the local maximum and the local minimum; and

the second sub-process comprises illuminating the structure withradiation having a central wavelength aligned with a midpoint betweenthe local maximum and the local minimum, within a range of 40% of thewavelength separation between the local maximum and the local minimum.

-   23. The method of any of clauses 15-22, wherein the sub-processes    comprise at least one sub-process configured to measure the first    property of the structure and at least one sub-process configured to    measure the second property of the structure.-   24. The method of clause 23, wherein the first property of the    structure comprises reflectivity and the second property of the    structure comprises overlay between different layers of the    structure.-   25. The method of clause 23 or clause 24, further comprising    determining an improved value of the second property of the    structure using a combination of the second property obtained using    a sub-process of the first measurement process and the second    property obtained using the second measurement process.-   26. The method of any preceding clause, wherein the plurality of    structures formed on the substrate are formed by a lithographic    process.-   27. A device manufacturing method, comprising

forming a plurality of structures on a substrate using lithography; and

measuring the plurality of structures using the method of any precedingclause.

-   28. A metrology apparatus for measuring a plurality of structures on    a substrate, the metrology apparatus comprising:

a first measurement system configured to perform a first measurementprocess, the first measurement process comprising individually measuringeach of the plurality of structures to measure a first property of thestructure;

a second measurement system configured to perform a second measurementprocess, the second measurement process comprising measuring a secondproperty of each of the plurality of structures; and

a controller configured to control the second measurement process suchthat a radiation property of radiation used to illuminate each structureduring the second measurement process is individually selected for thatstructure using the measured first property for the structure.

-   29. The apparatus of clause 28, wherein the controller is configured    to perform the individual selection of the radiation property for    the second measurement process for each structure based on a    previously measured correlation between the first property and a    choice of the radiation property for the second measurement process    that enables a performance of the second measurement process to be    higher than for other choices of the radiation property for the    second measurement process.-   30. The apparatus of clause 28 or clause 29, wherein the first    property of the structure comprises reflectivity.-   31. The apparatus of any of clauses 28-30, wherein the first    property of the structure comprises an effect of the structure on    the polarization of radiation scattered from the structure.-   32. The apparatus of any of clauses 28-31, wherein the second    property of each structure comprises overlay between different    layers of the structure.-   33. The apparatus of any of clauses 28-32, wherein the radiation    property for the second measurement process comprises a spectral    distribution of intensity.-   34. The apparatus of clause 33, wherein the spectral distribution of    intensity comprises one or both of central wavelength and bandwidth.-   35. The apparatus of any of clauses 28-34, wherein the radiation    property for the second measurement process comprises a polarization    of the radiation.-   36. The apparatus of any of clauses 28-35, wherein:

the first measurement system comprises a first radiation sourceconfigured to illuminate each structure with radiation; and

the second measurement system comprises a second radiation sourceconfigured to illuminate each structure with radiation, wherein thefirst radiation source is different from the second radiation source.

-   37. The apparatus of any of clauses 28-36, wherein the apparatus    comprises a focus measurement system comprising a focus sensor    configured to measure a focus of an optical system used by the    second measurement system, and the first measurement process uses an    output from the focus sensor.-   38. The apparatus of clause 37, wherein the first property of the    structure comprises reflectivity and the signal strength from the    focus sensor is used to determine the reflectivity.-   39. The apparatus of any of clauses 28-38, wherein the first    measurement system comprises one or more optical elements that are    also used by a focus measurement system configured to measure a    focus of an optical system used by the second measurement system,    wherein the one or more optical elements are not used by the second    measurement system.-   40. The apparatus of any of clauses 28-39, wherein the first    measurement system comprises a first radiation source configured to    illuminate each structure with broadband radiation and the    controller is configured to perform the individual selection of the    radiation property for the second measurement process for each    structure based on a spectroscopic analysis of the data from the    first measurement process.-   41. The apparatus of any of clauses 28-40, wherein the plurality of    structures formed on the substrate are formed by a lithographic    process.-   42. A lithographic system comprising:

a lithographic apparatus configured to form a plurality of structures ona substrate using lithography; and

the metrology apparatus of any of clauses 28-41 configured to measurethe plurality of structures formed by the lithographic apparatus.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic, and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. A method of measuring a plurality ofstructures formed on a substrate, the method comprising: obtaining datafrom a first measurement process, the first measurement processcomprising individually measuring each of the plurality of structures tomeasure a first property of the structure and the first measurementprocess comprising one or more sub-processes; calculating a sensitivityof each of one or more sub-processes of the one or more sub-processes toa second property of the structure; and using a second measurementprocess to measure a value of the second property of each of theplurality of structures, the second measurement process comprisingilluminating each structure with radiation having a radiation propertythat is individually selected for that structure using the measuredfirst property for the structure, wherein the individual selection ofthe radiation property for the second measurement process is performedusing one or more of the calculated one or more sensitivities.
 2. Themethod of claim 1, wherein the individual selection of the radiationproperty for the second measurement process is performed for eachstructure based on a previously measured correlation between each of oneor more of the calculated one or more sensitivities and a choice of theradiation property for the second measurement process that enables aperformance of the second measurement process to be higher than forother choices of the radiation property for the second measurementprocess.
 3. The method of claim 1, wherein the individual selection ofthe radiation property for the second measurement process comprises aselection of a central wavelength of a spectral distribution ofintensity for the second measurement process.
 4. The method of claim 1,wherein the individual selection of the radiation property for thesecond measurement process comprises a selection of a polarizationproperty for the second measurement process.
 5. The method of claim 1,further comprising individually selecting a polarization property ofreflected radiation detected during the second measurement process usingone or more of the calculated one or more sensitivities.
 6. The methodof claim 1, wherein the one or more sub-processes comprise two or moreselected from: illuminating the structure with radiation having a firstpolarization property and detecting reflected radiation having a secondpolarization property; illuminating the structure with radiation havingthe second polarization property and detecting reflected radiationhaving the first polarization property; illuminating the structure withradiation having the first polarization property and detecting reflectedradiation having the first polarization property; and/or illuminatingthe structure with radiation having the second polarization property anddetecting reflected radiation having the second polarization property,wherein the first polarization property is different to the secondpolarization property.
 7. The method of claim 6, wherein the firstpolarization property is orthogonal to the second polarization property.8. The method of claim 1, wherein: the one or more sub-processescomprise a first sub-process and a second sub-process; a wavelengthdependence of the sensitivity of the first sub-process to the secondproperty of the structure is substantially the same as the wavelengthdependence of the second sub-process to the second property of thestructure and comprises a local maximum and a local minimum; the firstsub-process comprises illuminating the structure with radiation having acentral wavelength aligned with the local maximum or the local minimum,within a range of 10% of the wavelength separation between the localmaximum and the local minimum; and the second sub-process comprisesilluminating the structure with radiation having a central wavelengthaligned with a midpoint between the local maximum and the local minimum,within a range of 40% of the wavelength separation between the localmaximum and the local minimum.
 9. The method of claim 1, wherein the oneor more sub-processes comprise at least one sub-process configured tomeasure the first property of the structure and at least one sub-processconfigured to measure the second property of the structure.
 10. Themethod of claim 9, wherein the first property of the structure comprisesreflectivity and the second property of the structure comprises overlaybetween different layers of the structure.
 11. The method of claim 9,further comprising determining an improved value of the second propertyof the structure using a combination of the second property obtainedusing a sub-process of the first measurement process and of the secondproperty obtained using the second measurement process.
 12. The methodof claim 1, wherein the plurality of structures formed on the substrateare formed by a lithographic process.
 13. A device manufacturing method,comprising: forming a plurality of structures on a substrate usinglithography; and measuring the plurality of structures using the methodof claim
 1. 14. A non-transitory computer-readable medium comprisinginstructions therein, the instructions, upon execution by a computersystem, configured to cause the computer system to at least: obtain datafrom a first measurement process, the first measurement processcomprising individually measuring each of a plurality of structures on asubstrate to measure a first property of the structure and the firstmeasurement process comprising one or more sub-processes; calculate asensitivity of each of one or more sub-processes of the one or moresub-processes to a second property of the structure; and configure asecond measurement process to measure a value of the second property ofeach of the plurality of structures, the configured second measurementprocess comprising illuminating each structure with radiation having aradiation property that is individually selected for that structureusing the measured first property for the structure, wherein theindividual selection of the radiation property for the secondmeasurement process is performed using one or more of the calculated oneor more sensitivities.
 15. The computer-readable medium of claim 14,wherein instructions are configured to cause performance of theindividual selection of the radiation property for the secondmeasurement process for each structure based on a previously measuredcorrelation between each of one or more of the calculated one or moresensitivities and a choice of the radiation property for the secondmeasurement process that enables a performance of the second measurementprocess to be higher than for other choices of the radiation propertyfor the second measurement process.
 16. The computer-readable medium ofclaim 14, wherein the individual selection of the radiation property forthe second measurement process comprises a selection of a centralwavelength of a spectral distribution of intensity for the secondmeasurement process and/or a selection of a polarization property forthe second measurement process.
 17. A metrology system, comprising: ameasurement system configured to perform the first and secondmeasurement processes; the non-transitory computer-readable medium ofclaim 14; and a controller configured to control the second measurementprocess such that a radiation property of radiation used to illuminateeach structure of a plurality of structures on a substrate during thesecond measurement process is according to the individually selectedradiation property for the second measurement process.
 18. A metrologyapparatus comprising: a measurement system configured to at least:perform a first measurement process, the first measurement processcomprising individually measuring each of a plurality of structures on asubstrate to measure a first property of the structure and the firstmeasurement process comprises one or more sub-processes; and perform asecond measurement process, the second measurement process comprisingmeasuring a value of a second property of each of the plurality ofstructures; and a controller configured to calculate a sensitivity ofeach of one or more sub-processes of the one or more sub-processes to asecond property of the structure and control the second measurementprocess such that a radiation property of radiation used to illuminateeach structure during the second measurement process is individuallyselected for that structure using the measured first property for thestructure, wherein the individual selection of the radiation propertyfor the second measurement process is performed using one or more of thecalculated one or more sensitivities.
 19. The metrology apparatus ofclaim 18, wherein a first measurement apparatus is used to perform thefirst measurement process and a second different measurement apparatusis used to perform the second measurement process.
 20. A lithographicsystem comprising: a lithographic apparatus configured to form aplurality of structures on a substrate using lithography; and themetrology apparatus of claim 18 configured to measure the plurality ofstructures formed by the lithographic apparatus.